1. Field of the Invention
The present invention relates to a process for purifying an aqueous hydrogen peroxide solution containing impurities, some of which are in ionic form, in which the said aqueous solution is purified by passing it through at least one first resin of anionic type in order to remove at least some of the anionic impurities, and then through a second resin of cationic type in order to remove at least some of the cationic impurities, or vice versa.
2. Description of the Related Art
Hydrogen peroxide is generally manufactured by autoxidation of an anthraquinone derivative or of a mixture of such derivatives. The said anthraquinone derivative(s) is(are) used dissolved in a complex mixture of organic solvents, such as an aromatic hydrocarbon mixed with an ester or an alcohol. This solution constitutes the working solution. This working solution is first hydrogenated in the presence of a catalyst, which brings about the conversion of the quinones into hydroquinones. It is then oxidized by placing it in contact with air or oxygen-enriched air. During this oxidation, the hydroquinones are reoxidized into quinones, with simultaneous formation of hydrogen peroxide. The said hydrogen peroxide is extracted with water and the working solution undergoes a regeneration treatment before being reused.
Since the solubility in the aqueous phase of the organic compounds present is not entirely zero, the aqueous hydrogen peroxide solution thus obtained can contain up to 1000 mg/l of dissolved organic compounds. These compounds are either in the form of ions or in the form of nonionic molecules. In general, acetate and formate ions form the majority of the organic anions.
Various techniques can be used for the purification of such a solution, such as liquid-liquid extraction, distillation, crystallization, passage through adsorbent resins and/or ion-exchange resins, reverse osmosis, filtration, ultrafiltration, etc.
Generally, such an aqueous solution, obtained by autoxidation of at least one anthraquinone derivative, undergoes a first distillation intended in particular to concentrate it. This first distillation is performed, in principle, at the same hydrogen peroxide production site and is carried out in metal plants, generally aluminium plants. On contact with these metal plants, the aqueous hydrogen peroxide solution becomes charged with metal ions from the wear of the surfaces of the said plants with which it is placed in contact. This contamination with metal ions nevertheless remains minimal with respect to the contamination with organic anions which is inherent in the autoxidation process described previously.
The hydrogen peroxide solution thus produced after having been distilled once is referred to as being of IPG ("Industrial Pure Grade") quality. It can be used directly in the food and pharmaceuticals industries. It can also be purified to reach a higher quality intended in particular for the electronics industry, for cleaning the surface of silicon wafers on which integrated circuits are subsequently produced.
A person skilled in the art differentiates the grades VLSI, ULSI and SLSI whose concentration for each cation is less than 100, 10 and 1 ppb, respectively. An ultra-pure grade whose concentration for each cation is less than 0.1 ppb has recently appeared.
Purification starting with an IPG-grade solution is generally carried out in several steps and includes a second distillation followed by passage(s) through ion-exchange resins, or even in addition at least one passage through adsorbent resin. The said distillation is carried out in glass columns, which minimizes the risks of metal pollution. This second distillation has the effect of lowering the concentrations of cations and anions in the hydrogen peroxide solutions, in particular the concentration of acetate ions. This is due to the vapour pressure of acetic acid, which is about 8 times as high as that of hydrogen peroxide at 20.degree. C.